I. Field of the Invention
This invention relates, in general, to a method for the improved recovery of metal from ore by heap leaching and, more specifically, to an improved method employed especially where the heaps are relatively wide or closely spaced to one another.
II. Description of the Prior Art
Hydrometallurgical work done during the last two decades have included efforts to separate metals, for example gold and silver, from gangue material or host rock, by leaching with a solvent, such as cyanide solutions. Much of the early work was done by the Reno Metallurgy Research Center of the U.S. Bureau of Mines in Reno, Nevada. This is further explained in a paper entitled "Silver Extraction from Marginal Resources" by H. J. Heinen, D. G. Peterson and R. E. Lindstrom, delivered at the annual meeting of the American Institute of Mining, Metallurgical & Petroleum Engineers, in New York City, New York on Feb. 16, 1975.
In the extraction of metals from metal-bearing ore by leaching, the recovery of metals is generally far inferior to the recovery rates achieved by other known methods, for example roasting and flotation. One reason for this inferior recovery is the inability to efficiently heap leach rock which has been finely ground since the leaching solution cannot satisfactorily filter through the densely packed ore particles. On the other hand, leaching ore crushed to a size common to heap leaching, for example about two to five centimeters, results in poor and uneven penetration of the leaching solution into the crushed ore. This inferior penetration is due in part to the larger size of the crushed ore pieces as well as to the stationary position of the crushed ore.
In heap leaching, the leaching solution will normally pursue a downward path of least resistance through the rock pile or heap. This phenomenon is known as channeling and results in uneven leaching action. It is possible, for the purpose of improving recovery, to rearrange the ore and divert channeling by setting off small explosive charges within the leach pile. This approach, however, has the drawback of producing fines, i.e., small rock particles which are densely packed and impair the filtering down of the leaching solution.
In co-pending application, Ser. No. 133,516 there is described an improved method of leaching metal bearing ores, wherein removable, void-forming members are placed near the bottom of the ore body and removed during the leaching process, causing partial internal collapse and shifting of the ore, thus exposing additional ore surfaces to the effect of the leaching solution. While this process has many advantages, it also has certain limitations. For example, in certain instances where ore bodies to be heap leached are relatively wide or closely spaced, it may be difficult, if not impossible, to pull the void-creating members out in a substantially horizontal direction towards the sides of the heap. This is the case when the heap or ore body is either continually widened by adding ore to its sides or where the width of the heap is such that the void-creating members have to be so long that the friction to be overcome in pulling them out would be excessive. It is also the case where the heaps are so closely spaced that there is insufficient room between neighboring heaps to pull void-creating members out in a substantially horizontal direction.
It is, therefore, an object of the present invention to provide a method for leaching metal bearing ores which is devoid of the above-noted disadvantages.
It is another object of this invention to provide a method for leaching metal bearing ores wherein improved distribution and percolation of the leaching material through the ore body is achieved.
It is still another object of this invention to provide a method for leaching metal bearing ores wherein void-creating members may be used in conjunction with relatively wide or closely spaced heaps of ore.
It is another object of the present invention to provide a method for leaching metal bearing ores wherein pulling out of the void-creating members is made easier.
It is a further object of this invention to provide a method for leaching metal bearing ores wherein void-creating members, such as pipes, can be limited to a length which does not result in friction with the ore which cannot be overcome by standard earth moving equipment.
It is yet another object of the present invention to provide a method for leaching metal bearing ores wherein the void-creating members can be pulled out in an upward direction by standard earth moving equipment.
It is still another further object of this invention to provide a method for leaching metal bearing ores wherein the ore near the bottom of the heap, which is packed more densely and which is affected more by the channeling effect that the ore near the top of the heap, comes in contact with a greater proportion of the leaching solution used.
It is still another object of this invention to provide a method for leaching metal bearing ore wherein a greater proportion of leaching solution, not weakened in its ability to dissolve additional metal by first filtering through the top portion of the heap, reaches the ore nearer to the bottom of the heap.
It is yet a further object of this invention to provide a method for leaching metal bearing ore wherein void-creating members having thinner walls are employed.